The invention concerns the desalting of solutions with the use of ion exchangers. It finds application principally in the desalting of water during the preparation of make-up water for steam generators, but is of general use in the energy generation and chemical industries.
The desalting of water is generally carried out by means of ion exchangers. In the first step of such processes in which the water is freed of cations, the solution is normally contacted by a strongly acidic cation exchanger in the form of hydrogen. After this, the anions of strong acids are removed by means of a weakly basic anion exchanger in the form of a free base. If desired, these two exchangers can be mixed together.
Desalting operations of this general type are often made reversible. The salts present in the water are first contacted with the strongly basic anion exchanger in the form of a hydroxide, a carbonate, or a hydrogen carbonate, and the corresponding hydroxide, carbonate, or hydrogen carbonate compounds resulting from such reaction are then neutralized by means of a weakly or strongly acidic cation exchanger in the form of hydrogen.
Theoretically, in such known processes ion exchangers need one equivalent of mineral acid and one equivalent of base (e.g., sodium hydroxide, ammonium hydroxide, or sodium carbonate), are necessary to remove equivalent of salt from the water. In practice, it has been found that for the removal of one equivalent of salt, 1.2-3 equivalents of mineral acid and 1.3-2 equivalents of base must be used. This drawback results in a high operating cost of the desalting process; moreover, it generates as a by-product waste water having a significant salt concentration. In fact, in such processes it has been found that for each equivalent of salt removed, approximately 1.5-4 equivalents units of neutral salts in addition are produced in the waste water. For these reasons much effort has been devoted to study of the methods of regenerating ion exchange resins by other means than those employing chemicals, for example, by heat energy (the so-called "Sirotherm process"), by electrical energy (electrodialysis), or mechanical energy (piezodialysis). But because of many technical problems resulting from the use of other than chemical means, none of these processes has been adapted to industry.